Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 041741
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
141 PM EDT WED MAY 4 2016

.AVIATION...

UPPER LOW AND SURFACE LOW WERE SPINNING OVER SOUTHWEST LOWER
MICHIGAN THIS AFTERNOON AND WILL SLOWLY EXIT SOUTHEAST OVERNIGHT.
IN THE MEANTIME SEVERAL BAND OF SHOWERS WERE SPIRALLING AROUND
THESE LOWS ACROSS SOUTHEAST MI. THERE WERE EMBEDDED THUNDERSTORMS
TOO. EXPECT THIS TO CONTINUE THROUGH THE AFTERNOON...BUT SHOW A
DECREASE TOWARD 00Z AS BOTH THE LOWS SPIN AWAY AND INSTABILITY
DECREASES. WINDS WILL BE STRONG ENOUGH AROUND THIS LOW EARLY...10
TO 15 KTS AND THE DIRECTION WILL BE DIFFICULT GIVEN THE LOW
PASSING CLOSE TO THE DETROIT AREA TAFS. ALSO EXPECT SOME MVFR
CEILINGS AND VISIBILITIES IN THE SHOWERS.

SOME SHOWERS WILL LINGER INTO THE EVENING BUT BY 03Z ALL TAF SITES
WILL BE DRY. CEILINGS FROM 4000 TO 6000 FEET EXPECTED THROUGH MUCH
OF THE NIGHT. SOME SUGGESTION OF STRATUS WITH RECENT RAINS AND
NORTHEAST FLOW OFF OF THE GREAT LAKES AND HAVE A TEMPO GROUP FOR
THE NORTHERN TAF SITES AROUND DAYBREAK. THURSDAY WILL HAVE MORE
DRYING WITH SCATTERED CUMULUS AND A NORTH WIND AROUND 10 KTS.

FOR DTW...THUNDER THREAT HAS ALREADY ROTATED NORTH AND EAST OF DTW
FOR THE AFTERNOON. STILL BANDS OF SHOWERS WILL MOVE IN FOR MVFR
VISIBILITIES AND CEILING THROUGH THE AFTERNOON. AFTER 02Z SHOWERS
END AND CEILINGS IMPROVE TO 4000 TO 6000 FEET AND BECOME SCATTERED
BY DAYBREAK.

//DTW THRESHOLD PROBABILITIES...

* MEDIUM IN CEILINGS AT OR BELOW 5000 THROUGH 05Z.

* LOW IN THUNDERSTORMS IMPACTING THE AREA THROUGH 21Z.

&&

.PREV DISCUSSION...ISSUED 1236 PM EDT WED MAY 4 2016

UPDATE...
WATER VAPOR IMAGERY INDICATES UPPER ENERGY IS STARTING TO CLOSE
OFF OVER SOUTHWEST MICHIGAN AS PREDICTED BY NWP SUITE. THIS SHOULD
PIVOT THE NORTH-SOUTH ORIENTATED BAND OF PRECIPITATION THAT WAS
OVER WESTERN MICHIGAN THIS MORNING MORE INTO SOUTHEAST LOWER
MICHIGAN AS CYCLONIC FLOW INCREASES AROUND THE SYSTEM AND THE
DEFORMATION BAND TAKES ON A MORE NORTHWEST TO SOUTHEAST
ORIENTATION. THIS IS ALREADY STARTING TO TAKE SHAPE PER LATEST
RADAR IMAGERY. BAND OF HEAVIEST RAIN IS EXPECTED TO STRETCH
ROUGHLY FROM THE SAGINAW VALLEY DOWN THROUGH THE DETROIT AREA THIS
AFTERNOON BEFORE DROPPING SOUTH EARLY TONIGHT AS THE LOW CENTER
WORKS SOUTHWARD. SOME MINOR ADJUSTMENTS WERE MADE TO PRECIPITATION
PLACEMENT BASED ON RADAR TRENDS AND LATEST RUNS OF HIGHER-RES
MODELS (HRRR...NAM12...NMM)...WITH HIGHEST POPS AND QPF SHIFTED
VERY SLIGHTLY TO THE WEST AND SOUTH. BIGGEST CHANGES WERE IN THE
FORM OF HIGHER POPS OVER LENAWEE COUNTY...LOWER POPS OVER THE
THUMB...AND A MORE PRONOUNCED DRYING TREND OVER THE NORTHERN HALF
OF THE FORECAST AREA EARLY TONIGHT.

STRONGER SHOWERS AND THUNDERSTORMS HAVE ALSO STARTED TO DEVELOP
EARLY THIS AFTERNOON AS WARMING OCCURS WITH DIURNAL HEATING IN THE
LOW LEVELS AND THE UPPER LEVELS COOL IN THE PROXIMITY OF THE
DEVELOPING UPPER LOW. THIS HAS RESULTED IN A FEW HUNDRED J/KG OF
MU CAPE PER LATEST SPC ANALYSIS. STILL NOT EXPECTING SEVERE
WEATHER...BUT LOCALLY HEAVY RAINFALL IS EXPECTED.

PREV DISCUSSION...ISSUED 337 AM EDT WED MAY 4 2016

DISCUSSION...

IMPRESSIVE SPREAD STILL EXISTS AMONG THE GUIDANCE THIS MORNING AS
FORECAST LEAD TIME DIMINISHES TO ZERO. WATER VAPOR IMAGERY SHOWS A
POWERFUL LEAD WAVE DIGGING INTO THE MIDWEST WITH A NEARLY
EQUIVALENTLY STRONG TRAILING WAVE OVER WEST-CENTRAL WISCONSIN AT
07Z. THE TRAILING WAVE HAS BEEN POORLY RESOLVED BY MANY MEMBERS OF
THE GUIDANCE...THE RAP APPEARING TO HANDLE IT THE BEST. EVEN SO, THE
RAPID REFRESH IS STRUGGLING TO KEEP UP WITH 3-HOUR PRESSURE FALLS OF
3-5MB THIS MORNING.

EXPANDING SHOWER COVERAGE IN ADVANCE OF THE WAVE IS OCCURRING WITHIN
THE CORRIDOR OF MOISTURE TRANSPORT THAT HAS EMERGED OVER SOUTHWEST
MICHIGAN DURING THE LAST COUPLE OF HOURS. FOR NOW, THIS ACTIVITY IS
STRUGGLING TO PENETRATE THE CWA DUE TO ANTECEDENT DRY AIR EVIDENT ON
THE 00Z DTX SOUNDING. COVERAGE IS EXPECTED TO INCREASE IN SE
MICHIGAN OVER THE NEXT SEVERAL HOURS AS THE COLUMN SATURATES, LARGE
SCALE FORCING INCREASES, AND DIURNAL DESTABILIZATION GETS UNDERWAY.

THE PRIMARY FEATURE OF INTEREST, THE BAND OF RAIN WITH EMBEDDED
HEAVY SHOWERS, IS SO FAR BEHAVING AS ANTICIPATED. ITS NARROW
STRUCTURE IS ATTRIBUTABLE TO DEEP LAYER LINEAR/CONVERGENT FORCING
ALONG THE STRONG TROUGH AXIS. THE UPPER TROUGH WILL BEGIN THE
PROCESS OF CLOSING OFF LATE THIS MORNING OR EARLY THIS AFTERNOON AS
ENERGY CONTINUES TO FLOOD SOUTH INTO THE WESTERN GREAT LAKES. AS
THIS OCCURS, THE BAND WILL STALL AND PIVOT IN CYCLONIC FASHION WITH
PRECIPITATION RATES INTENSIFYING AS DEFORMATION FORCING RAMPS UP
WEST OF US-23. IN ADDITION, DIURNAL HEATING AND STEEP LAPSE RATES
WILL CONTRIBUTE TO A LOW DENSITY CAPE PROFILE CHARACTERIZED BY A FEW
HUNDRED J/KG OF POTENTIAL ENERGY. SKINNY CAPE WITHIN PERSISTENT
STRONG DEFORMATION WILL BE SUFFICIENT TO PRODUCE A SOLID HALF INCH
TO THREE QUARTERS OF RAIN OVER WESTERN AREAS. A LOCALIZED AREA -
PERHAPS JUST THE FOOTPRINT OF A COUNTY OR TWO -COULD EASILY SEE OVER
AN INCH IN THIS SETUP. MOST LIKELY AREA FOR HEAVIEST RAIN WILL BE
BETWEEN LIVINGSTON AND BAY COUNTIES WEST OF US-23/I-75. THE LACK OF
SIGNIFICANT MOISTURE (PWAT < 0.80") AND A VERY SHALLOW (< 5KFT) WARM
CLOUD LAYER WILL PUT A DEFINITE CEILING ON POTENTIAL RAINFALL
AMOUNTS SO THINGS WILL NOT GET OUT OF CONTROL.

UNCERTAINTY IN PRECIP POTENTIAL INCREASES FOR EASTERN AREAS,
INCLUDING DETROIT METRO, AS THE CONVERGENT RAINFALL BAND STALLS UPON
THE MID-LEVEL LOW CLOSING OFF. HOWEVER, LATEST RAP SIMULATIONS SHOW
RESPECTABLE CAPE OF A FEW HUNDRED J/KG ACROSS THE EAST. WILL LIKELY
SEE SOME SUBTLE RIDGING FORCED OVER THE EASTERN CWA DURING PEAK
HEATING, BUT LOW/MID-LEVEL THETA MAY BE JUST ENOUGH IN THE STEEP
LAPSE RATE ENVIRONMENT TO SERVE AS A TRIGGER FOR DISORGANIZED SHOWER
ACTIVITY DURING THE AFTERNOON. THUS, LIKELY POPS REMAIN IN PLACE IN
SPITE OF THE WESTWARD ADJUSTMENT FROM YESTERDAY MORNING`S FORECAST.

WARMING TREND INTO THE 60S THURSDAY AND LOW 70S FRIDAY AS RIDGING IS
FORCED THROUGH THE CWA DURING THE LATE WEEK PERIOD. LINGERING
CYCLONIC FLOW AND MOISTURE WILL KEEP A PARTIAL COVERAGE OF DIURNAL
CU AND POSSIBLY JUST AN ISOLATED SHOWER IN THE FORECAST THROUGH
THURSDAY. DEEP LAYER SUBSIDENCE WILL SUPPORT FULL SUN AND MUCH
WARMER TEMPS ON FRIDAY AS THE BOUNDARY LAYER TAPS INTO 850MB TEMPS
IN THE HIGH SINGLE DIGITS BY AFTERNOON.

MARINE...

MODEST NORTHERLY WINDS WILL BEGIN TO TAKE HOLD ACROSS THE CENTRAL
GREAT LAKES TODAY AS LOW PRESSURE PIVOTS THROUGH LOWER MICHIGAN.
SLIGHT INCREASE IN NORTHERLY FLOW TONIGHT INTO EARLY THURSDAY MAY
LEAD TO A BRIEF PERIOD OF GUSTS NEAR 20 KNOTS ACROSS SOUTHERN
SECTIONS OF LAKE HURON DURING THIS TIME.  THE ONSHORE FLOW WILL ALSO
BRING A PERIOD OF ELEVATED WAVE CONDITIONS ALONG THE THUMB DURING
THIS TIME.  HOWEVER SMALL CRAFT ADVISORY LEVEL CONDITIONS ARE
CURRENTLY NOT ANTICIPATED.  HIGH PRESSURE WILL THEN BRING TRANQUIL
MARINE CONDITIONS FOR THE LATE WEEK PERIOD.

HYDROLOGY...

STRENGTHENING LOW PRESSURE WILL DROP SOUTHEAST ACROSS THE STATE
THROUGH THIS EVENING.  A PERIOD OF STRONG FORCING ASSOCIATED WITH
THIS SYSTEM WILL SLIDE INTO WESTERN PORTIONS SE MICHIGAN LATE TODAY.
THIS WILL FOCUS AN AXIS OF HEAVIER RAINFALL ALONG HIGHWAY 23
CORRIDOR...WHERE RAINFALL AMOUNTS FROM ONE HALF TO THREE QUARTERS OF
AN INCH ARE EXPECTED.  DECREASING RAINFALL AMOUNTS WITH EASTWARD
EXTENT...LEAVING EASTERN PORTIONS OF METRO DETROIT NORTHWARD INTO
THE THUMB WITH LESS THAN A QUARTER OF AN INCH.  THE BULK OF THIS
RAINFALL WILL OCCUR DURING THE AFTERNOON AND EARLY EVENING HOURS.
RAINFALL WILL TAPER OFF QUICKLY EARLY TONIGHT WITH GENERALLY DRY
WEATHER THURSDAY AND FRIDAY.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MICHIGAN WATERS OF LAKE ERIE...NONE.
&&

$$

AVIATION.....RBP
UPDATE.......HLO
DISCUSSION...JVC
MARINE.......MR
HYDROLOGY....MR


YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE
AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).



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